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   <h2 class="title">预定义常量</h2>

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    PHP 向它运行的任何脚本提供了大量的<a href="reserved.constants.html" class="link">预定义常量</a>。不过很多常量都是由不同的扩展库定义的，只有在加载了这些扩展库时才会出现，或者动态加载后，或者在编译时已经包括进去了。
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